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bremms
Carpal Tunnel
*****

Reged: 08/31/12

Loc: SC
Re: 120mm ED refractor for planets - wrong choice? new [Re: Jon_Doh]
      #6251881 - 12/13/13 10:57 PM

8" F6 with decent mirrors and a 25% obstruction will eat an 120mm ED for lunch. Proper collimation is a must but it's a no contest. 6" F8 with a 20% obstruction? Still better but close. A 100-120mm scope is better in poor seeing. My 4" F15 Jaegers is often better than my 6" F10 Jaegers on poor nights. Poly strehl on the 6" F10 isn't great either. My 8" F6 newt is a good bit better on good nights with proper cool down. (0.98 Strehl)

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Jon_Doh
professor emeritus


Reged: 09/16/11

Loc: On a receiver's back
Re: 120mm ED refractor for planets - wrong choice? new [Re: bremms]
      #6253087 - 12/14/13 05:47 PM

Quote:

8" F6 with decent mirrors and a 25% obstruction will eat an 120mm ED for lunch. Proper collimation is a must but it's a no contest. 6" F8 with a 20% obstruction? Still better but close. A 100-120mm scope is better in poor seeing. My 4" F15 Jaegers is often better than my 6" F10 Jaegers on poor nights. Poly strehl on the 6" F10 isn't great either. My 8" F6 newt is a good bit better on good nights with proper cool down. (0.98 Strehl)




Not where I live.


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Eddgie
Postmaster
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Reged: 02/01/06

Re: 120mm ED refractor for planets - wrong choice? new [Re: Eric63]
      #6253415 - 12/14/13 08:56 PM

I loved your MTF plots. I won't get into which scope would be best for you. To me, telescope performance differs far more off axis than on axis, and things like true field and ergonomics to me are actually perhaps more important than CA or central obstruction.

But your plots were beautiful. You clearly grasp the key concept of the MTF plot, which is to show how contrast for different size detail will be affected by the aperture and by things like CO, CA, or optical quality.

It is a fascinating topic, and it has always pained me that more people have not made an effort to really understand the basics of the Modulation Transfer Function.

So, a tip of my hat to you. You have clearly studied the topic and have been able to apply it to the real world and presented a graphic that was easy to understand and hopefully educational for anyone that took the time to study it.

I sure enjoyed seeing it....


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